STANDARD ISOTOPE VERSUS NITROGEN BALANCE CRITERIA FOR ASSESSING THE EFFICIENCY OF NITROGEN SOURCES FOR BARLEY

Abstract

Nitrogen balance (fertilizer N accounted for in the soil–plant system) and standard isotope (obtained on above-ground plant parts) criteria were used to evaluate the efficiency of nitrogen sources for barley grown on a Chernozemic and a Solonetzic soil under greenhouse conditions. The isotope criteria, percent total N in the plant tissue derived from fertilizer (% N d.f.f.), "A" values, and uptake of fertilizer N by the crop, clearly indicated the superiority, in terms of plant availability, of the NO 3 − -N source, followed by NH 4 + -N, with urea the least effective. In contrast, loss of nitrogen from the soil–plant system was greatest for the NO 3 − -N and least for the urea (i.e., 67 vs. 26% on the Solonetzic soil). Such conflicting results can be explained on the basis of slow hydrolysis of the urea and rapid plant uptake of N from the NO 3 − -N form. It is concluded that, although isotope-derived criteria such as % N d.f.f., A values, and uptake by the crop of fertilizer N provide precise measurements of the performance of N sources, serious errors in causative factors may be made unless "nitrogen balance" data are available. The significance of primary and corrected (rate of fertilizer N application corrected for fertilizer N loss) A values are discussed.